3 Myths About Smartphone Batteries That Need to Die

You've probably had a friend — or even the smartphone salesperson — tell you it's bad for the battery to leave the device plugged in overnight. But on the other hand, who wants to wake up at 3 a.m. to unplug their phone?

Relax, my friend. Your phone's battery is smarter than you think. When you plug in your smartphone at night, the lithium-ion battery begins to slowly recharge until it is full or "saturated" at 4.1 volts.

"Then it turns off," says Isidor Buchmann, founder and CEO of Cadex Electronics and the creator of the educational website Battery University. "It's as if it were on the shelf and not connected at all."

So, if the charger is functioning correctly, it's impossible to "overcharge" your phone's battery beyond its limit of 4.1 volts. But is it bad for the battery to be fully charged for long periods? That depends on what you expect out of a phone battery.

"If somebody wants to keep a battery forever, then you could call it 'overcharging,'" says Bachmann. "But with a consumer product like a smartphone, people don't care about battery life. In two to three years the glass breaks and you buy a new one and the old battery still has some life in it."

Technologies with longer lifespans, like satellites and electric cars, are a different story. In that case, engineers must take special precautions to extend the life of lithium-ion batteries. The rechargeable batteries in electric cars, for example, don't charge to 100 percent full capacity or drain all the way to zero.

"They work in the middle where the battery would have the least amount of stress over time," says Bachmann.

So you don't need to wake up at night and unplug your fully charged phone. The whole point of charging a smartphone is to maximize the amount of time you can use it before plugging it back in. You want it at a 100 percent in the morning so that it lasts all (or most of) the day.

Nope, it's better to plug your phone in at intervals throughout the day than to let the battery run completely down before recharging. Forty to 80 percent capacity is the sweet spot. As it says on the Battery University website, "Similar to a mechanical device that wears out faster with heavy use, the depth of discharge (DoD) determines the cycle count of the battery. The smaller the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses." (The DoD refers to how much energy a battery has delivered. In a fully charged battery, DoD is 0 percent; in a 70 percent charged battery, it's 30 percent). 

However, once every three months, a battery should be calibrated by letting it run down until the "low battery" light appears and recharging fully.

Actually, heat is a much bigger threat to battery longevity than your charging practices. Leaving your phone on a sunny windowsill or the dashboard of your car is a guaranteed way to drain its capacity.

"It's like a carton of milk," says Bachmann, who literally wrote the book on rechargeable batteries. "If it's kept out of the fridge it doesn't last as long. With a lithium-ion battery, it's corrosion that sets in and deteriorates the battery."

Interestingly, the combination of excessive heat and a full charge can be problematic if you're storing a lithium-ion battery for a long time between uses. Battery University ran tests on lithium-ion batteries stored for a full year at different temperatures.  A battery stored at 100 percent charge at 40 degrees C (104 degrees F) will lose 35 percent of its total capacity over a year. Under the same conditions, a battery stored with only a 40 percent initial charge will lose just 15 percent of its capacity after three months.

That's why folks in the battery industry store and ship batteries under controlled temperatures and never at full charge. Again, does that mean you should keep your smartphone in the fridge overnight? No, that would be crazy, unless you don't plan to use it for a year or you just like the feel of a cool iPhone in the morning.

A certain degree of capacity loss is inevitable with all lithium-ion batteries, Bachmann explains, and no single factor — fully charging, fully depleting, too many cycles, too much heat — will run your battery into the ground. But all of these together will definitely take their toll. The good news is that your kid will drop the phone in the toilet long before the battery burns out.